The present invention relates to an organic electrolytic cell, which has a high capacity and high voltage and is superior in charge and discharge characteristics and safety.
In recent years, a secondary cell wherein an electrically conductive polymer, an oxide of a transition metal or the like is used as the positive electrode, and metallic lithium or a lithium alloy is used as the negative electrode has been proposed as a cell to be used in place of Nixe2x80x94Cd storage cells and lead storage cells, because of its high energy density.
However, when such a secondary cell is subjected to repeated charge and discharge, its capacity is largely lowered due to deterioration of the positive or negative electrode, and thus there still remains a problem in its practical aspect. Particularly by deterioration of the negative electrode, mossy lithium, called dendrites, are formed, and through repeated charge and discharge, the dendrites finally pierce the separator and cause a short circuit. In some case, the cell is broken and thus there has been a problem in safety, too.
To solve the above problems, there has been proposed a cell wherein a carbon material such as graphite is used as the negative electrode and a lithium-containing metallic oxide such as LiCoO2 is used as the positive electrode. This cell is a so-called rocking chair-type cell wherein, after assembly of the cell, lithium is supplied from the lithium-containing metallic oxide as the positive electrode to the negative electrode through charge, and lithium of the negative electrode is returned to the positive electrode through discharge. Although the cell is characterized by a high voltage and high capacity, the high energy density as an advantage of the lithium cells has not been obtained.
In the above rocking chair-type cell, it is an essential feature to use the lithium-containing metallic oxide as the positive electrode. Therefore, when using metallic oxides (e.g. V2O5, MnO2, TiS2, etc.), metallic sulfides, electrically conductive polymers (e.g. polyacene organic semiconductor, etc.) or the like proposed as the positive electrode material for lithium secondary cells, it is necessary to previously carry lithium on the positive or negative electrode. To obtain these positive electrode materials, there is required a method of carrying lithium, practically and simply.
In recent years, with the rapid progress of a study about a negative electrode material capable of reversibly carrying lithium, a material capable of carrying lithium in the amount exceeding that of C6Li, which is a theoretic amount of the carbon material, and an oxide such as SnO2 and SiO2 have been proposed as the negative electrode material for high-capacity lithium secondary cells. Among them, an infusible and insoluble substrate having a polyacene skeletal structure and a hydrogen/carbon atomic ratio of 0.50 to 0.05, the substrate being a heat-treated product of an aromatic condensation polymer, is capable of doping with lithium up to C2Li (Synthetic Metals, 73 (1995) P273). However, the above locking chair-type cell wherein this infusible and insoluble substrate is used as the negative electrode and the lithium-containing metallic oxide as the positive electrode can attain a capacity higher than that in the case of the carbon material after assembly, but there still remains an unsatisfactory respect in its capacity.
To solve the above problems, PCT Publication No. WO95/8852, whose application was filed by the present applicant, has proposed an organic electrolytic cell comprising a positive electrode, a negative electrode and a solution of lithium salt in an aprotic organic solvent as an electrolytic solution, wherein the positive electrode contains a metallic oxide, the negative electrode is an infusible and insoluble substrate having a polyacene skeletal structure and a hydrogen/carbon atomic ratio of 0.50 to 0.05, the substrate being a heat-treated product of an aromatic condensation polymer, and the total amount of lithium contained in the cell is not less than 500 mAh/g and the amount of lithium originating in the negative electrode is not less than 100 mAh/g, based on the infusible and insoluble substrate as the negative electrode. Although this cell can attain a high capacity, a method of carrying lithium originating in the negative electrode, practically and simply, is required in the case of assembly of a practical cell such as cylindrical-type cell. Various specific methods thereof are proposed in Japanese Patent Kokai (Laid-Open) Publication Nos. 162159/1996, 162160/1996, 162161/1996 and 255633/1996. However, any of these methods has a problem in uniformity and operating property and the problem has still to be completely solved at present. That is, a most simple method in these specific methods includes a method of attaching a lithium metal on a positive or negative electrode, inserting the resultant into a cell container, together with the positive or negative electrode and a separator, pouring an electrolytic solution and allowing to stand, thereby to carry lithium on the positive or negative electrode. However, this method had such a problem that, since a lower limit of the thickness of a lithium metal foil to be attached, which can be mass-produced, is about 30 xcexcm, the thickness of the positive and/or negative electrodes increases thereby to restrict design of the cell and to exert an influence particularly on charge and discharge characteristics.
The present inventors have studied intensively in light of the problems described above, thus completing the present invention. An object of the present invention is to provide an organic electrolytic cell, which is easy to produce, and which has a high capacity and high voltage and is superior in charge and discharge characteristics and safety.
Still another objects, features and advantages of the present invention will become apparent from the following description.
To attain these objects, the organic electrolytic cell of the present invention has the following construction. That is, the present invention provides an organic electrolytic cell comprising a positive electrode, a negative electrode and a solution of lithium salt in an aprotic organic solvent as an electrolytic solution, wherein a current collector of the positive electrode and a current collector of the negative electrode are respectively provided with pores piercing from the front surface to the back surface, an active material of negative electrode is capable of reversibly carrying lithium, and lithium originating in the negative electrode is carried by electrochemical contact with lithium arranged to face the negative or positive electrode and an opposed area of said lithium is not more than 40% of an area of the negative electrode.
The present invention also provides an organic electrolytic cell comprising a positive electrode, a negative electrode and a solution of lithium salt in an aprotic organic solvent as an electrolytic solution, wherein a current collector of the positive electrode and a current collector of the negative electrode are respectively provided with pores piercing from the front surface to the back surface, an active material of the positive electrode and an active material of the negative electrode are capable of reversibly carrying lithium, and at least one portion of lithium originating in the positive electrode is carried by electrochemical contact with lithium arranged to face the negative or positive electrode and an opposed area of said lithium is not more than 40% of an area of the positive electrode.